This application claims benefit of priority under 35 U.S.C. 119(a) from Italian Application No. M198A002829 filed Dec. 24, 1998, the entire contents of which are incorporated by reference herein.
1. Field of the Invention
The present invention relates generally to bistable actuators and more particularly to a low-energy bistable actuator for residual current devices such as for example residual-current circuit breakers. The present invention will be described with particular reference to a residual-current circuit breaker without intending in any way to limit its scope of application.
2. Discussion of the Background
A residual-current circuit breaker is an automatic circuit breaker for AC circuits which opens when the vector sum of the currents in the conductors of the circuit, which is zero in normal conditions, exceeds a preset value.
Residual-current circuit breakers are meant to prevent metallic parts, such as the enclosures of appliances and the metallic masses of the protected area, connected to an earth system apparatus of suitable resistance, from becoming live (indirect protection) and, if sufficiently sensitive and fast-acting, can in certain conditions also provide protection against contact with normally live parts (direct protection).
The main characteristic of residual-current circuit breakers is that they achieve extremely short circuit opening and closure times.
A residual-current circuit breaker is shown generically in FIG. 1. The circuit breaker comprises a sensor 4 for detecting the residual fault current, generally constituted by a magnetic core through which the neutral 2 and the phase 3 pass and are then input connected to a load (not shown), and by a secondary winding 10 across which a voltage is generated when the fault current is present. An electronic coupling circuit 5, which is supplied by the voltage generated across the secondary winding, is provided between the current sensor 4 and an actuator 6.
The residual-current circuit breaker further comprises a release or disengagement mechanism 7 which is actuated by the actuator 6 and opens the contacts 8 and 9 if there is an earth fault current.
Conventional actuators used in residual-current circuit breakers are of the electromagnetic type, e.g. electromagnetic relays of the demagnetization type, and have the following drawbacks. They are sensitive to magnetic fields which consequently alter their operation. For example, the presence of an electromagnetic field can cause an unwanted and unwarranted actuation of the actuator, or an external magnetic field can alter the magnetization of the electromagnetic relay, consequently altering the sensitivity of the residual-current circuit breaker. These actuators are also sensitive to impacts and vibrations, are mechanically complicated to provide, and are expensive.
Accordingly, one object of the invention is to provide a bistable actuator for residual-current devices which is insensitive to external magnetic fields.
Another object of the present invention is to provide a bistable actuator for residual-current devices which provides assured intervention with extremely short actuation times.
Another object of the present invention is to provide a bistable actuator for residual-current devices, which requires a low triggering energy, so as to make the actuator applicable in self-powered residual-current circuit breakers wherein the small amount of energy is derived exclusively from the earth fault current.
Yet another object of the present invention is to provide a bistable actuator for residual-current devices which is substantially insensitive to impacts and vibrations.
Still yet another object of the present invention is to provide a bistable actuator for residual-current devices, which is highly reliable, relatively easy to manufacture and at competitive costs.
These objects and others are achieved according to the present invention by providing a novel bistable actuator for residual-current devices including a lamina having at least one face, a first end, and a second end, and configured to actuate from a first stable position to a second stable position; at least one layer of piezoelectric material provided on the at least one face of the lamina at the first end of the lamina, and configured to stimulate the lamina to shift from the first stable position to the second stable position in response to an electrical activation signal; and a support element comprising an interlock coupling, and configured to support the first end and the second end of the lamina, wherein the interlock coupling is configured to interlock the first end of the lamina to the support element, and the second end of the lamina is rigidly coupled to the support element in order to keep the lamina in the first stable position.